(1) Field of the Invention
The present invention relates to a system and method for predicting a failure in a machine tool and especially relates to the suitable system and method for predicting the failure in a numerical control (NC) machine tool.
(2) Description of the background art
Generally, a failure monitoring system is incorporated into a numerical control machine tool. The failure monitoring system monitors whether the numerical control (NC) machine tool malfunctions or fails, issues automatically an alarm when the numerical control (NC) machine tool malfunctions, and simultaneously halts the NC machine tool.
Such a monitoring system as described above always monitors a monitor data such as a load current flowing through a servomotor and which is varied according to a working situation of the machine tool through a series of working processes and issues the alarm when the monitor data exceeds a common alarm threshold level during the execution of the series of working processes of the machine tool.
It is noted that during the execution of the series of working processes the monitor data such as the load current are often varied over a predetermined range due to a variation of load although the machine tool operates normally.
Therefore, in a previously proposed monitoring system as described above, the common alarm threshold level is necessarily set to a relatively large value. Consequently, if a detection of abnormality in the machine tool is delayed, a catastrophic damage might occur in an installation including the machine tool.
On the other hand, a Japanese Patent Application First Publication No. Showa 63-14205 exemplifies another previously proposed failure monitoring system for the machine tool which determines whether the monitor data exceeds the common alarm threshold level, while avoiding the situations of deriving the monitor data when the large variation in the monitored data generally occurs such as when the machine tool is started and halted. However, since the alarm level is also set so as to be common to the series of working processes of the machine tool in the above-described case, the problem of delaying the detection of an abnormality is still present.